In this study, four knee prostheses from a single family of products, each one characterized by a different constraint level, were analyzed via the finite element method in order to compare their performances in terms of contact areas as well as von Mises tibial stress in the bone and in the insert.
From the results obtained with the simulations, it emerged that the mobile bearing inserts tend to be more compliant in the anterior part, exhibiting a higher contact area in this latter.
Innocenti [9] reported the same tendency regarding the ultra-congruent insert contact area with its value in each configuration being significantly higher than those of the fixed bearings. Thus, the surface of the MB ultra-congruent insert is designed to be more compliant and therefore have a maximal contact between the femoral component and the insert.
Shiramizu et al. [40] highlighted a similar contact area for the FB PS insert in the first configuration with two prostheses coming from different manufacturers respectively: the NexGen LPS-flex fixed and the JOURNEY, with slightly different contact areas in the last two configurations. In this study, those differences thus resulted from all the knee models being analyzed at the different angles of flexion under the same loading condition: a vertical axial load of 3600 N. In addition, differences in the manufacturing design features also influenced the outcome for the contact areas.
Hofer et al. [41] reported the same trend for the contact areas with higher values for the CR prosthesis compared to the PS prosthesis. Besides, this study showed higher medial contact areas compared to the lateral ones for the CR prosthesis. This trend was, however, not observed for the PS prosthesis. Nonetheless, this study only investigated one configuration similar to ours: a squatting position at 90° of flexion.
The results of the present study showed that the distribution of the contact areas was significantly different for the mobile and fixed-bearing models in the different loading conditions, but it remains in an acceptable range. It is finally to be highlighted that the different models analyzed exhibited a symmetrical medial and lateral distribution of the contact areas, which is not always common among all the currently available prostheses (which followed the approach of the "medial pivot" design) [42].
In addition, the main relevant difference in the average tibial bone stresses was previously observed between the mobile and fixed-bearing models. The proximal and distal tibial bone stresses obtained with the different designs were similar to the ones obtained in other studies [28, 29]. Besides, by increasing the level of constraint, higher stress values were observed along the tibial bone as demonstrated in the literature by the comparison between a conventional and semi-constrained PS [13, 43].
Sathasivam and Walker [44] suggested that increased frontal plane conformity reduced subsurface stresses. However, it was not demonstrated in this study as the difference in the resulting tibial stresses between the more conforming prostheses and the others were not important enough.
Considering the results obtained and the comparison among the different models reported here, the most important finding that can be highlighted concerns the ultra-congruent mobile bearing model: This design indeed appeared to be the one able to provide the highest congruency in terms of the contact area when compared to the other prostheses analyzed, and these higher values guarantee, therefore, an overall more homogeneous stress distribution in the insert. These results, together with the literature [9], suggest thus that this design would be able to provide the best performance in terms of kinetics, in the case that the level of constraint was suitable for the patient involved (in terms of soft tissue configuration present).
It is then remarkable to note that, regardless of the studied configuration, the distribution of the stress in the different regions of interest of the tibia (proximal and distal) was not remarkably different among all the models. Changing the prosthetic implant would therefore not induce a big variation in the tibial stress distribution. However, it would remarkably change the distribution of stress at the interface between the prosthetic components (tibial insert and femoral component) and therefore this factor is the main one to take into consideration in the decision-making process.
Limitations
There are various limitations associated with this study, mainly related to assumptions made during the implementation of the FE models.
Soft tissues (such as the muscles or some ligaments) were not incorporated into the different models, but their contribution was nonetheless considered as an influence on the loading conditions applied [35]. Another assumption was to simplify the collateral ligaments by modeling them as beams. This is, however, a common approach in the literature and it can be found in previously validated ligaments models [9, 24,25,26, 31, 34, 45]. Therefore the validity of this study is not affected. All the organic material models (bony structures as well as the soft tissues) were further assumed to be linear elastic and homogeneous, although it is well-known that the cortical and cancellous bone present spatial inhomogeneity in their properties. However, such assumption, in the finite element approach for this kind of studies and for this reason, was considered acceptable [9, 25, 26, 29, 34]. Another simplification present in the study was to consider the behavior of the polyethylene as linear elastic, without taking into account the plastic region. Therefore, this approximation led to an overestimation of the local value of the polyethylene stress. However, this overestimation served a further purpose as it allowed for analysis of the eventual worst-case scenario and the results obtained showed that in no case were the critical stress values reached. It also should be considered that the goal of this study was to provide a comparison between the different studied models using the same approach [9, 25, 26], and, therefore, the fact that the same approximation is done for every model does not present an issue. The next limitation lies in the geometries used to describe the different structures: The bone models used, indeed, did not take into consideration any variation of the bone anatomy and bone deformity that could alter the final TKA outcome [9, 34, 46, 47], but this ideal approach is largely used for finite element modeling in the biomechanical field [24, 26, 48, 49] and, therefore, the study on the effect of deformities can be seen as an eventual follow-up research project. Furthermore, the list of prostheses analyzed included only one family of products, and therefore they were not comprehensive and not representative of the whole models available. However, this choice allowed for comparison of the influence of constraint of various levels only and not involving any other design features, thus representing a viable way to perform the study.
Of note, the patello-femoral joint was not included in the model, since it was not the focus of the analysis. This choice was made by taking into account the fact that the forces involved in this joint in the configurations analyzed were negligible compared to the ones involved in the considered regions of interest. Furthermore, this choice was made in agreement with previously validated models addressing similar configurations [9, 24, 28].
Finally, the positioning and the alignment of the prosthetic components were assumed to be ideal during the simulations, ignoring the possible postoperative misalignments occurring during TKA surgeries [9, 50]. Also in this case, since the main aim was to conduct a comparative study, the fact that all the models were ideally implanted should not be considered as an issue.